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Evidence in radar, reflectance, and visible imagery indicates that surface and subsurface water ice is present inside permanently shadowed regions in the north polar region of Mercury. The origin of this ice and the time at which it was delivered to the planet are both unknown. Finding the smallest, most easily eroded ice deposits on Mercury can help answer these questions. Here we present evidence for volatiles trapped in cold traps of scales ∼ 1-10 m. We consider two possible delivery methods for these deposits: a gradual, slow accumulation by micrometeorites or solar wind implantation and an episodic deposition, either primordial or by a recent comet impact. We conclude that the mechanism that best explains the presence of volatiles in these micro cold traps is a comet impact that most likely occurred in the last ∼ 100 Ma.Plain Language Summary Craters near the north pole of Mercury, one of the warmest planets in the solar system, cast persistent shadows that are so cold they can trap water ice for billions of years. Although many observations show that ice is present inside these polar cold traps, its age and the way it was delivered to the planet are not well known. Here we present evidence for the presence of ice deposits on scales of 1-10 m, the smallest discovered so far, near the north pole of Mercury. Due to the relative shallowness of these micro cold traps, an episodic deposition, such as a comet impact, is a more probable delivery mechanism than an ongoing slow accumulation by micrometeorites. Using previously estimated surface erosion rates, we find that the age of this ice is most likely lower than 100 million years.Lately, it was hypothesized that ice may persist in micro cold traps that form in PSRs cast by craters and random small-scale topographic features (Hayne & Aharonson, 2015). In this case, the term micro does not mean Key Points: • Using MLA data and a thermal illumination model, we find evidence for ice trapped in micro cold traps of scales < 10 m and thickness < 1 m • Surface micro cold traps occupy 1%-2% of the polar region, compared to ∼ 7% occupied by the larger cold traps • A recent comet impact is more likely to explain our findings than continuous delivery by, for example, solar wind implantation or micrometeorites

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The morphology of isolated barchan dunes on Mars and Earth may shed light on the dynamic conditions that form them, their migration direction and the physical properties of the sediments composing them. Prior to this study, dune fields have been largely analyzed manually from aerial and satellite imagery, as automatic detection techniques are often not sufficiently accurate in outlining dunes. Here, we employ an instance segmentation neural network to detect and outline isolated barchan dunes on Mars and Earth. We train and test the model on martian targets using Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) images, and find it sufficiently accurate (mAP=77% on the test dataset) to characterize dune field dynamics. Using our trained model we detect and map the global distribution of barchan dunes relative to previously mapped dune fields, and find that barchan dunes are more abundant in the northern hemisphere than in the southern hemisphere. These contrasting abundances of barchans may reflect latitudinally dependent wind regimes, sediment supply, or sediment availability.

Solar irradiance dominates the heat flux incident on airless planetary bodies. In thermal equilibrium, surface roughness affects the temperature distribution by changing the incidence angle local to each slope. In order to simulate temperatures and thermal emissions at different phase angles, existing thermophysical models usually employ computationally expensive techniques such as ray tracing. Here we derive the equilibrium surface temperature distribution of sunlit Gaussian rough surfaces, providing an exact solution for the Sun at the zenith and an approximate solution for the general case. We find that although the slope distribution of realistic airless surfaces is often non‐Gaussian, their temperature distribution is well modeled assuming a Gaussian slope distribution. We additionally present closed‐form expressions that describe the radiation emitted from rough surfaces at different emissions angles and employ them to radiometrically estimate the roughness of the lunar surface using measurements obtained by Lunar Reconnaissance Orbiter (LRO) Diviner. Our model may also be applied to studying the roughness of resolved and unresolved surfaces on other airless planetary bodies.

Sand mobilized by wind forms decimeter-scale impact ripples and decameter-scale or larger dunes on Earth and Mars. In addition to those two bedform scales, orbital and in situ images revealed a third distinct class of larger meter-scale ripples on Mars. Since their discovery, two main hypotheses have been proposed to explain the formation of large martian ripples—that they originate from the growth in wavelength and height of decimeter-scale ripples or that they arise from the same hydrodynamic instability as windblown dunes or subaqueous bedforms instead. Here we provide evidence that large martian ripples form from the same hydrodynamic instability as windblown dunes and subaqueous ripples. Using an artificial neural network, we characterize the morphometrics of over a million isolated barchan dunes on Mars and analyze how their size and shape vary across Mars’ surface. We find that the size of Mars’ smallest dunes decreases with increasing atmospheric density with a power-law exponent predicted by hydrodynamic theory, similarly to meter-size ripples, tightly bounding a forbidden range in bedform sizes. Our results provide key evidence for a unifying model for the formation of subaqueous and windblown bedforms on planetary surfaces, offering a new quantitative tool to decipher Mars’ atmospheric evolution.

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